The demand for lithium ion secondary battery is increasing rapidly for its use as a power source for electric automobile, hybrid automobile, electronic appliances, etc. With respect to the negative electrode material of the lithium ion secondary battery, the main stream is graphite particles.
The graphite used as the negative electrode material of the lithium ion secondary battery can be largely divided into natural graphite and artificial graphite. The graphite particles obtained by grinding natural graphite have a scaly (sheet-like) shape and show striking anisotropy caused by the crystal structure.
That is, natural graphite has a crystal structure in which a large number of AB planes each having large area are laminated in the C axis direction. In natural graphite particles, the thickness of lamination in C axis direction is small as compared to the area of AB plane; consequently, the natural graphite particles have a scaly shape as a whole.
Meanwhile, artificial graphite particles can be produced so as to have a nearly spherical shape by selecting the production method therefor. Artificial graphite particles can also be produced so as to have a crystal structure low in anisotropy.
For example, there can be produced spherical graphite particles in which a plurality of disc-like graphites of different radii are orientated randomly and laminated, or there can be produced columnar graphite particles in which a plurality of disc-like graphites of same radius are laminated with the AB planes being arranged in parallel.
Such artificial graphite particles are, however, generally expensive and low in crystallinity. Artificial graphite produced so as to have a high crystallinity is close to natural graphite in properties. Accordingly, the graphite particles obtained by grinding an artificial graphite of high crystallinity has a scaly or complex shape, similarly to natural graphite.
The negative electrode of lithium ion secondary battery is generally constituted by a collector (e.g. copper foil) and a thin graphite layer formed on the surface of the collector.
The graphite layer is preferred to have a high density in order to allow the lithium ion secondary battery to have large charge and discharge capacities. Ordinarily, the high density is achieved by compressing the graphite layer formed on the surface of the collector, by pressing, rolling, etc.
However, when the graphite layer of natural graphite particles or highly crystalline artificial graphite particles is compressed by pressing or rolling, the graphite particles undergo a compressive force and the planes (AB planes) of graphite particles are orientated so as to be in parallel to the compression plane. This is caused because the graphite particles have a thin scaly shape.
That is, individual scaly graphite particles constituting the graphite layer tend to be orientated so that each AB plane becomes parallel to the surface of the collector. Such orientation of graphite particles in molded material (e.g. graphite layer) is hereinafter referred to simply as “orientation”.
Orientation of graphite particles is not preferable in the graphite layer constituting the negative electrode of battery. The orientation of graphite particles occurring on the surface of electrode by pressing makes difficult the infiltration of electrolytic solution into the graphite layer of electrode. As a result, the site of contact between graphite and electrolytic solution is restricted to around the surface of graphite layer, inviting the reduction in power generation ability of battery.
In the graphite layer of battery, electricity flows to the thickness direction of graphite layer. This thickness direction agrees with the C axis direction of the graphite particles orientated in the graphite layer. The conductivity of graphite crystal is large in the AB plane direction and small in the C axis direction. For this reason, when graphite particles are orientated, the graphite layer has a large electric resistance, resulting in a small charge and discharge capacity of battery.
Meanwhile, artificial graphite particles of low crystallinity are low in orientation but is small in charge and discharge capacities per unit mass. Accordingly, it is not preferred to use such artificial graphite particles as a negative electrode material of lithium ion secondary battery.
Other conventional graphite electrodes also have, in many cases, the above-mentioned orientation problem of graphite particles.
The present inventors proposed a method for producing primary spheroidized graphite particles in order to obtain highly crystalline graphite particles low in anisotropy caused by crystal structure (Patent Literature 1).
In order to produce a negative electrode material of low anisotropy using primary spheroidized graphite particles, there was disclosed a method of kneading primary spheroidized graphite particles and a graphitizable binder (e.g. pitch), subjecting the kneaded material to pressure molding, firing the pressure-molded material, and graphitizing the fired material (Patent Literature 2). In this method, high-temperature firing of 2,800° C. is conducted in the final graphitization step. Such high-temperature firing, however, involves various problems in production, unlike in low-temperature firing.